648 THE AMERICAN NATURALIST. [VoL. XXXII. 
the most extensive and biologically notable event in geological 
history. In its effects on life, whether indirect or direct, it was 
of vastly greater significance than any period since, for con- 
temporaneous with, and as a consequence of, this revolution 
was the incoming of the new types of higher or terrestrial 
vertebrates. Through the researches, now so familiar, in the 
field and study, of the two Rogerses, of Dana, and of Hall, 
we know that all through the Paleozoic era at least some 
30,000 to 40,000 feet of shoal water sediments, both marine 
and fresh water, derived from the erosion of neighboring lands, 
were accumulated in a geosynclinal trough over the present site 
of the range extending from near the mouth of the St. Law- 
rence to northern Georgia. At the end of the era ensued a 
series of movements of the earth’s crust resulting from the 
weight of this vast accumulation, which in a geologically brief 
period sank in, dislocated, and crushed the sides of the trough, 
and folded the strata into great close parallel folds, besides 
inducing more or less metamorphism. These folds rising from 
a plateau formed mountain ranges perhaps as high as the Sierra 
Nevada or Andean Cordillera of the present day. The plateau 
emerged above the surface of the Paleozoic ocean, and was 
carved and eroded into mountain peaks, separated by valleys 
of erosion, the rivers of the Appalachian drainage system 
cutting their channels across the mountain ranges. 
But this process of mountain building and erosion was not 
confined to the end of the Paleozoic era. Willis! has shown 
that there have been several successive cycles of denudation, 
covering a period extending from the end of the Paleozoic era 
to the present time. And it is the fact of these successive 
cycles of denudation both on the Atlantic and Pacific slopes 
of our continent that is of high significance to the zoologist 
from the obvious bearings of these revolutions on the produc- 
tion of variations. Indeed it is these phenomena which have 
suggested the subject of this address. 
We can imagine that this great plateau, in the beginning of 
the Mesozoic era, with its lofty mountain ranges and peaks 
rising from the shores of the Atlantic, presented different 
1 National Geographical Magazine, vol. i (1889), pp. 291-300. 
